In steps in the production of semiconductor devices such as microprocessors, memories, and CCDs or of flat panel display devices such as TFT liquid-crystal displays, a circuit pattern or thin film of submicron size is formed on surfaces of substrates made of silicon (Si), silicon oxide (SiO2), glass, or another material. It is extremely important to reduce the contaminants present in a slight amount on the substrate surface in each step in the production.
Among such contaminants, fine particles such as, e.g., silica particles, alumina particles and particles of organic substances especially reduce the yield of devices. It is therefore necessary to reduce these fine particles as much as possible before the substrate is sent to the subsequent step. The technique generally employed for the removal of such contaminants is to clean the substrate surface with a cleaning liquid medium.
It is generally known that alkaline solutions are effective in the removal of particulate contaminants. Alkaline aqueous solutions such as aqueous solutions of ammonia, potassium hydroxide, tetramethylammonium hydroxide, or the like are used for the surface cleaning of silicon or SiO2 substrates for semiconductor devices or glass substrates for display devices. Cleaning with a cleaning liquid medium comprising ammonia, hydrogen peroxide, and water (referred to as “SC-1 cleaning liquid medium” or “APM cleaning liquid medium”) is also used extensively (this cleaning is referred to as “SC-1 cleaning” or “APM cleaning”) (see, for example, W. Kern and D. A. Puotinen, RCA Review, p.187, June (1970)). The cleaning generally requires from 1 to 15 minutes.
Typical apparatus for the cleaning are of two kinds. One of these is a batch cleaning apparatus in which two or more substrates placed in a cassette are cleaned usually by immersion in a cleaning liquid medium placed in a cleaning tank. The other is a sheet-by-sheet (so-called single wafer) cleaning apparatus in which one substrate is attached to a holder and a cleaning liquid medium is sprinkled over the substrate surface usually while revolving the substrate (e.g., when the substrate has a disk form, it is revolved in the circumferential direction). The batch cleaning apparatus can treat a large number of substrates per unit time (has a high throughput). However, this apparatus not only has a large size but also has problems, for example, that the contaminants which have been released from the device formation-side surface of a substrate or from the back side thereof may readhere to the device formation-side surface of another substrate to cause the so-called cross contamination and that it is necessary to use the cleaning liquid medium in a large amount even when only one substrate is cleaned.
On the other hand, the sheet-by-sheet cleaning apparatus has a small size and is free from cross contamination. However, it has a problem that the throughput is low because substrates are cleaned one by one.
Recently, as a result of the trend toward higher fineness in circuit patterns, particles smaller than those which have been regarded as contaminants have become problematic. Particles have the property of becoming difficult to remove as the particle diameter thereof decreases. It has been pointed out that the APM cleaning is insufficient in the ability to remove fine particles having a particle diameter on a 0.1 μm level (see H. Morinaga, T. Futatsuki, and T. Ohmi, J. Electrochem. Soc., Vol.142, p.966 (1995)). In device production, a further improvement in throughput and a further increase in production efficiency are desired and to reduce the cleaning time also is an important subject.
APM cleaning has been used for more than 30 years in order to remove particulate contaminants. At present, however, the particle diameters of the particles which should be removed are far smaller and the production efficiency required is far stricter as compared with those at that time when this cleaning method was developed. There is a desire for a cleaning method which can more efficiently remove fine particles to highly clean the substrate surface in a shorter time period.
On the other hand, addition of various surfactants to an alkaline cleaning liquid medium has been proposed for the purpose of, e.g., inhibiting the substrate surface from being roughened or etched, improving wetting properties, or improving the ability to remove oily contaminants or particulate contaminants.
For example, Japanese Patent Laid-Open No. 335294/1993 proposes a technique for inhibiting a substrate surface from being roughened. This technique comprises adding a surfactant to an alkaline solution of hydrogen peroxide and thereby regulating the solution so as to have a contact angle of 10° or smaller with semiconductor substrates.
Japanese Patent No. 3,169,024 proposes a technique for improving the substrate surface-wetting properties of a cleaning liquid medium. This technique comprises adding an ethylene oxide addition type nonionic surfactant which has a polyoxyethylene group and the number of ethylene oxide (oxyethylene) groups in the polyoxyethylene group is from 3 to 10 to an alkaline cleaning liquid medium containing hydrogen peroxide.
Japanese Patent Laid-Open No. 2001-40389 proposes to add various surfactants to an alkaline aqueous solution in order to inhibit the silicon substrate from being etched.
Japanese Patent Laid-Open No. 121418/1999 proposes a cleaning liquid medium for semiconductor substrates which contains a specific surfactant so as to have the improved ability to remove, in particular, oily contaminants.
Japanese Patent Laid-Open No. 245281/1995 proposes to add an alkylbenzenesulfonic acid to an alkaline cleaning liquid medium containing hydrogen peroxide in order to improve the ability to remove contaminants. Furthermore, Japanese Patent Laid-Open No. 251416/1993 proposes to add a fluorochemical surfactant comprising a fluoroalkyl-sulfonamide compound to an APM cleaning liquid medium in order to improve the ability to remove particles.
However, even when those known surfactants are actually added to an APM cleaning liquid medium for the purpose of more efficiently removing fine particles on a 0.1 μm level in a shorter time period, which is the recent subject as stated above, then no improvement is obtained in the ability to remove particles on a 0.1 μm level or use of the cleaning liquid medium thus obtained poses problems because it produces the following side effects (1) to (4).    (1) The surfactants in the cleaning liquid mediums separate out as oil droplets at room temperature or elevated temperatures. The cleaning liquid mediums thus become milk-white. They have reduced cleaning performance and use of these cleaning liquid mediums results, for example, in residual oil droplets on the substrate surface.    (2) The cleaning liquid mediums froth considerably and adversely influence the operation of the cleaning apparatus.    (3) The surfactants themselves remain on the substrate surface.    (4) The surfactants are substances which may adversely influence the natural environment and there is no appropriate method of treatment for waste liquids resulting from the cleaning.